The present invention relates generally to x-ray reflectors, and more particularly to a wavelength-selective wavetrap for reducing unwanted longer-wavelength background reflections from a mirror designed for high reflectivity in a chosen x-ray band of the electromagnetic spectrum.
The ALEXIS (Array of Low Energy X-Ray Imaging Sensors) satellite is designed to survey the entire sky in three narrow wavelength bands (186, 171, and 133 .ANG.) with a spectral resolution of about 5% and a spatial resolution of 0.5.degree.. Each telescope on the satellite has a field-of-view of 33.degree. and utilizes a metal multilayer mirror at near normal incidence (12.5.degree.-7.6.degree. from the normal) to focus cosmic ultrasoft x-ray and extreme ultraviolet light onto a curved microchannel-plate detector. Each mirror consists of 60 to 100 alternating layers of molybdenum and silicon. Unfortunately, there is a strong geocoronal line of He II at 304 .ANG., which causes severe background interference difficulties in the detection of the shorter wavelength radiation. Use of bandwidth-limiting filters in this region severely compromises performance since materials that absorb the 304 .ANG. radiation also significantly attenuates the wavelength of primary interest.
The design of selective-reflection, selective-suppression optical coatings that optimize both high reflectivity at a desirable wavelength and low reflectivity at an undesirable wavelength are well-known for wavelengths longer than 1200 .ANG.. For example, multiple-layer dielectric-metal optical coatings have been used extensively throughout the far ultraviolet region. For wavelengths shorter than 1200 .ANG., but longer than 900 .ANG., transmissive spacing layers such as magnesium fluoride and lithium fluoride, and scattering layers such as aluminum, aluminum oxide, gold, silicon, and silicon dioxide have been found useful, as is described in ("Reflection/Suppression Coatings For 900-1200 .ANG. Radiation," by Jerry Edelstein, SPIE Vol. 1160, X-Ray/EUV Optics For Astronomy And Microscopy (1989), pages 19-25.
In the extreme ultraviolet, ultrasoft x-ray region, which is of great interest for astrophysical research, thin layers of materials having large differences in optical properties are employed. As stated above, molybdenum and silicon are two such materials. One multiple-layer mirror design having alternate molybdenum and silicon layers that reduce 304 .ANG. reflection simply decreases the thickness of the molybdenum layers while keeping the spacing between successive molybdenum layers the same. When the molybdenum occupies less than about 20% of the spacing, the 304 .ANG. reflectivity is reduced to less than 10.sup.-3. However, the peak reflectivity is also reduced to about 70% of the attainable maximum.
Accordingly, it is an object of the present invention to provide a metal, multilayer mirror having maximum reflectivity at chosen wavelengths in the soft x-ray, extreme-ultraviolet region of the electromagnetic spectrum at roughly normal incidence, while suppressing undesirable longer wavelength radiation by means of an antireflection coating also constructed as part of the metal multilayer structure.
Additional objects, advantages and novel features of the invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.